In the United States a standard has been proposed for digitally encoded high definition television signals. This standard is essentially the same as the MPEG-2 standard, proposed by the Moving Picture Experts Group (MPEG) of the International Standards Organization (ISO). This standard is described in a draft internal standard (DIS) publication entitled "Information Technology--Generic Coding of Moving Pictures and Associated Audio, Recommendation H.626" ISO/IEC 13818-2 DIS, 3/94 which is available from the ISO and which is hereby incorporated by reference for its teaching on the MPEG-2 digital video encoding standard.
The MPEG-2 standard is actually several different standards. In MPEG-2 several different profiles are defined, each corresponding to a different level of complexity of the encoded image. For each profile, different levels are defined, each level corresponding to a different image resolution. One of the MPEG-2 "standards", known as Main Profile, Main Level is intended for encoding video signals conforming to existing television standards (i.e., NTSC and PAL). Another "standard", known as Main Profile, High Level, is intended for encoding high-definition television images. Images encoded according to the Main Profile, High Level standard may have as many as 1,152 active lines per image frame and 1,920 pixels per line.
The Main Profile, Main Level standard, on the other hand, defines a maximum picture size of 768 pixels per line and 567 lines per frame. At a frame rate of 30 frames per second, signals encoded according to this standard have a data rate of 13,063,680 pixels per second. By contrast, images encoded according to the Main Profile, High Level standard have a maximum data rate of 1,152 * 1,920 * 30 or 66,355,200 pixels per second. This data rate is more than five times the data rate of image data encoded according to the Main Profile Main Level standard. The standard proposed for HDTV encoding in the United States is a subset of this standard, having as many as 1,080 lines per frame, 1,920 pixels per line and a maximum frame rate, for this frame size, of 30 frames per second. The maximum data rate for this proposed standard is still far greater than the maximum data rate for the Main Profile, Main Level standard.
Because of this difference in data rates, it is unlikely that the equipment used to decode Main Profile Main Level encoded data will be sufficient to decode high-definition television signals that have been encoded according to the Main Profile, High Level standard.
To be optimally effective, the coding systems should be relatively inexpensive and yet have sufficient power to decode these digital signals in real time.
One compression method that is contemplated for use in images which are encoded according to the MPEG-2 standard is to compress the image horizontally so that an image having a relatively large number of pixels per line is encoded as an image having a smaller number of pixels per line. To recover an image which does not appear to be horizontally squeezed, the data representing the image is desirably resampled to regenerate the original number of samples per line.
Using existing techniques, a decoder may be implemented using a single processor having a complex design and operating at a high data rate to perform this function. This high data rate, however, would require very expensive circuitry, which would be contrary to the implementation of a decoder in consumer television receiver in which cost is a major factor.